1. Field of the Invention
This invention relates to apparatus and method for detecting fluid flow outside a casing in a wellbore employing stationary temperature sensors.
2. Description of Related Art
To prevent uncontrolled flow of fluid along a wellbore containing casing, a hydraulic seal must exist between the casing and the rock through which the well is drilled. If this hydraulic seal exists, the well is said to have mechanical integrity outside the casing.
In wells used to produce hydrocarbons, this seal is required to prevent loss of hydrocarbons from production of unwanted fluid along with the hydrocarbon. During the treatment of hydrocarbon-production wells by fracturing or other stimulation processes, this integrity is important to insure that treatment fluids are placed in the hydrocarbon-containing zone. In hydrocarbon storage wells, mechanical integrity outside the casing is required to prevent loss of stored product. Very important also is the requirement in waste disposal wells that the injected fluid not flow along the wellbore to pollute other zones penetrated by the well.
Wells are used for injecting a variety of fluids into the earth. In 1989, 245 hazardous-waste injection wells were in operation in the United States. In addition, there were about 120,000 enhanced-recovery wells in use in oil production and about 38,000 wells in use strictly for disposal of oil-field brine. (G. A. Stewart and W. A. Pettyjohn, "Development of a Methodology for Regional Evaluation of Confining Bed Integrity," EPA/600/2-89/038, July 1989). Underground injection control regulations of the United States Environmental Protection Agency require that new injection wells demonstrate mechanical integrity prior to operation and that all injection wells demonstrate such integrity at regular intervals. Mechanical integrity includes the condition of no significant fluid movement into an underground source of drinking water through vertical channels adjacent to an injection well bore (J. T. Thornhill and B. G. Benefield, "Injection Well Mechanical Integrity", EPA/625/9-89/007, February 1990).
Wells used for either production or injection usually are equipped with one or more strings of casing, the casing being slightly smaller in diameter than the drilled hole at the depth where the casing is placed. Portland cement is normally pumped down the casing and into the annulus outside the casing to seal the annulus, in a process called "primary cementing." The process to repair an annulus where a hydraulic seal was not achieved by primary cementing is called "squeeze cementing." To achieve successful squeeze cementing, the liquid to provide sealing must be injected into the flow channel behind the casing.
Normally, at least two strings of casing are provided in wells. The largest diameter casing in wells extends only to shallower depths in the earth and is called surface casing. Regulations normally require that the surface casing in all wells be set deep enough to penetrate all zones which may produce potable water. Cement slurry is usually pumped around the surface casing and back to the surface of the earth to protect these zones. After the cement has cured, a deeper hole is then drilled below the surface casing and a lower string of casing is cemented in place, which may be an intermediate string of casing. If it extends to the total depth of the well, it is called the production string of casing. Cement is often placed over only the lower part of the lower strings of casing. The annulus above the cement is filled only with drilling fluid, so there is a potential flow of fluids from zones above the cement upward to the higher casing string. In recent years, there has been increasing concern regarding contamination of zones in old wells where the surface casing was not set deep enough.
From the time a well is drilled and casing is cemented in-place for the lifetime of the well and even, at times, after the well is abandoned, there is a need to know if fluids are flowing anywhere outside the casing, either in the cemented or uncemented sections of the wellbore. This includes the surface casing, any intermediate casing and production casing. Means for monitoring such wells to determine continuously if flow is occurring is also a great need.
It has long been recognized in industry that the primary cementing of wells is a complex and not entirely successful process. Cement can fail to achieve mechanical integrity of the well outside the casing because cement does not displace all the drilling fluid present in the well when the cement slurry is pumped into the well or because the pressure in the cement declines between the time the slurry is placed in the well and the time the cement develops mechanical strength. The paper "Field Measurements of Annular Pressure and Temperature During Primary Cementing," by C. E. Cooke, Jr. et al, J. Pet. Tech., August, 1983, p. 1429-38, explains why cement often fails to prevent leakage along a wellbore.
A variety of apparatus and methods are used to determine if a well has mechanical integrity outside the casing. Such procedures are often referred to as "cased hole" or "production" logging. The most widely used logs, based on sonic measurements, include the "cement bond" log and its derivatives. This log provides measurements of a sonic wave passing along or through the wall of the casing or the cement. In the cement bond long, higher attenuation is thought to indicate cement in contact with the wall of the casing, from which it is inferred that a hydraulic seal is provided by the cement. These logs do not determine if a hydraulic seal actually exists outside the casing, however. Other logs include radioactive tracer logs, nuclear activation logs (oxygen activation), noise logs and logs to measure temperature inside the casing. In hydrocarbon production wells the sonic logs are often run in new wells to indicate the quality of the cement. Other logs are more often run when a problem is suspected in a production well. In injection wells in the U.S., regulations require that hazardous waste wells be tested for mechanical integrity annually and other injection wells be tested every five years. Often, a variety of logs will be required to satisfy the test for mechanical integrity in hazardous waste injection wells.
Several production logging methods have been tested at the facility of the Environmental Protection Agency. Tests of the oxygen activation log were reported by Thornhill and Benefield in "Detecting Water Flow Behind Pipe in Injection Wells," EPA/600/R-92/041, February, 1992. The report concludes that this log is an excellent technique for detecting flow in or behind pipe, although a number of limitations of the tool are also discussed. Interpretation of results may be difficult. Cost of running the tool is not given in the report, but such nuclear activation logs are known to require advanced and expensive techniques.
Temperature logs used in the past have commonly measured the temperature of fluids inside the casing. Temperature anomalies in the inside fluid of the order of 1 degree or more are used to infer flow of fluid having a different temperature, commonly gas cooled from expansion or cool injection fluid, outside the casing. This commonly-used temperature log has been described in many publications and company brochures.
A tool for measuring temperature at the inside of the casing wall was disclosed in U.S. Pat. No. 4,074,756. This tool was used to detect flow outside casing with greater sensitivity than the conventional temperature log. In this tool, two temperature sensors mounted 180 degrees apart on spring arms to contact the casing wall are rotated to slide around the circumference of the casing. Results from using the tool were described in the paper "Radial Differential Temperature (RDT) Logging--A New Tool for Detecting and Treating Flow Behind Casing," by C. E. Cooke, Jr., published in J. Pet. Tech., June, 1979, pp. 676-682. Mechanical problems with the tool limited its acceptance in industry, although it has been used in hundreds of wells since its introduction. Measurements with the RDT tool were sometimes difficult to interpret, particularly above the perforations in a well when the measurements were made with fluid flowing past the tool inside the casing.
A recent paper described a concept for monitoring mechanical integrity of wells inside casing, which is affected by leaks of casing, tubing and packers ("Application of the Continuous Annular Monitoring Concept to Prevent Groundwater Contamination by Class II Injection Wells," SPE 20691, Soc. of Pet. Engrs., 1990). No continuous monitoring method for mechanical integrity of wells outside casing is known.
There is a great need for improved logging apparatus and method to measure with high sensitivity the leakage of fluids outside the casing of all types of wells, including production wells, injection wells, storage wells and abandoned wells. This apparatus and method should also be applicable to monitor continuously for flow external to the casing in a well. Such apparatus and method should be versatile and adaptable to use in many applications and types of wells. Data should be available in real time, stored for later analysis or used to provide an alarm under specified conditions indicating lack of mechanical integrity. Methods for estimating rate of fluid flow outside casing are also needed in wells where flow is detected.